It relates more particularly to the processes for the manufacture of self-sealing compositions based on a diene elastomer, such as natural rubber.
For some years, tyre manufacturers have been making particularly great efforts to develop novel solutions to a problem dating from the very beginning of the use of wheels fitted with tyres of inflated type, namely how to allow the vehicle to continue its journey despite a significant or complete loss of pressure from one or more tyres. For decades, the spare wheel was regarded as the sole and universal solution. Then, more recently, the considerable advantages related to its possible removal have become apparent. The concept of “extended mobility” was developed. The associated techniques make it possible to run with the same tyre, according to certain limits to be observed, after a puncture or a fall in pressure. This makes it possible, for example, to drive to a breakdown point without having to halt, under often hazardous conditions, to fit the spare wheel.
Self-sealing compositions capable of making it possible to achieve such an objective, by definition capable of automatically ensuring, that is to say without any external intervention, the airtightness of a tyre in the event of perforation of the latter by a foreign body, such as a nail, are particularly difficult to develop.
In order to be able to be used, a self-sealing layer has to satisfy numerous conditions of a physical and chemical nature. In particular, it has to be effective over a very wide range of operating temperatures, this being the case over the whole of the lifetime of the tyres. It has to be capable of sealing the hole when the perforating article remains in place; when the latter is expelled, it has to be able to fill in the hole and to render the tyre airtight.
Numerous solutions have admittedly been devised but to date it has not really been possible to deploy them in vehicle tyres, in particular due to difficulties in manufacturing these self-sealing compositions and thus to their final cost price.
In particular, high-performance self-sealing compositions, based on natural rubber and on hydrocarbon resin as tackifying agent (or tackifier), have been described in Patents U.S. Pat. No. 4,913,209, U.S. Pat. Nos. 5,085,942 and 5,295,525. These compositions are characterized by the combined presence of a high content of hydrocarbon resin, always greater than 100 parts per 100 parts of solid elastomer (or phr), and of a large amount of elastomer in the liquid state, generally in the form of depolymerized natural rubber (molecular weight typically of between 1000 and 100 000).
First of all, such a high content of resin, apart from the fact that it may be harmful to the hysteresis and consequently to the rolling resistance of the tyres, requires a particularly lengthy and difficult kneading of the elastomer matrix.
The use of a large amount of liquid elastomer admittedly improves the fluidity of the composition but such a use is the cause of other disadvantages, in particular of a risk of the self-sealing composition creeping when used at a relatively high temperature (typically of greater than 60° C.) frequently encountered during the use of some tyres.
Another major manufacturing problem may also arise: in the absence of a reinforcing filler, such as carbon black (furthermore undesirable, in a known way, for this type of application), the composition exhibits weak cohesion. This lack of cohesion may be such that the adhesiveness of the composition, resulting from the high content of tackifying resin employed, is no longer compensated for and prevails. This then results in a risk of undesirable adhesive bonding to the mixing equipment, which is unacceptable under industrial processing conditions.